1. Field of the Invention
The present invention relates to a method for mitigating the solidification segregation of a casting produced by the continuous casting method and a cast ingot produced by the ingot-making method. Such segregation causes surface flaws, cracks, and other material defects in the final steel product.
2. Description of the Related Art
The segregation of solutes during continuous casting results in formation of surface flaws and cracks of the casting, thereby impairing the qualities of the final product. Mitigation of the solidification segregation has therefore been desired. Known methods for mitigating the segregation include: adding calcium into the molten steel; preliminarily decreasing, by refining of the molten steel, the amount of solute elements which cause detrimental segregation; and lessening the roll-distance of a continuous casting machine to suppress the bulging of a casting and electromagnetically stirring the melt to mitigate the central segregation.
It is known that, when a casting is hot-rolled without once cooling down to normal temperature after its solidification, considerable hot-embrittlement occurs during the hot-rolling and, therefore, surface flaws frequently form. In one conventional practice, therefore, ingots cast at the ingot-making yard or castings produced by the continuous casting machine are allowed to cool down to room temperature and then are preliminarily reheated in a reheating furnace or are allowed to cool down to room temperature, cleared of surface flaws, then charged into a heating furnace to be heated to the rolling temperature and then hot-rolled (c.f. for example, "Iron and Steel Handbook" Third Edition, edited by Japan Institute for Iron and Steel III (1) pp 120-143, especially pp 140-141, and pp 207-212). In any case, by the reheating and heating described above, elements which segregate in the casting or the like and result in cracks and flaws can be uniformly distributed. The heat treatment necessary for uniformly distributing the elements, however, takes a disadvantageously long time of from 2 to 10 hours and involves temperatures of from 1200.degree. C. to 1300.degree. C.
From the viewpoints of saving energy and labor, however, either direct rolling or hot-charge rolling is preferable.
In direct rolling, the casting is not allowed to cool down to room temperature, but is rolled directly after the continuous casting. In hot-charge rolling, the casting is charged in a heating furnace before cooling to room temperature and is then rolled.
Japanese Unexamined Patent Publication (Kokai) No. 55-84203 proposes a method for suppressing the surface cracks in direct rolling and hot-charge rolling. The method proposed by this publication involves subjecting the casting, after its melting and solidification (the primary cooling), to ultraslow cooling during a secondary cooling stage until the initiation of the hot-rolling.
This publication threw light, by a simulation experiment, on a particular temperature range of from 1300.degree. C. to 900.degree. C. wherein elements, such as phosphorus, sulfur, oxygen, and nitrogen, detrimental to the hot-workability of steels, segregate and precipitate as non-metallic inclusions, and drew attention to the fact that surface cracks frequently occur when the percentage of reduction in area of steel materials becomes less than 60%. The method proposed in this publication controls the morphology of the above-mentioned elements precipitated as non-metallic inclusions so as to suppress the hot-cracking of castings.
Japanese Unexamined Patent Publication No. 55-109503 and No. 55-110724 also disclose slowly cooling the continuous castings prior to the hot-rolling and directly rolling them.
Japanese Examined Patent Publication (Kokoku) No. 49-6974 discloses a cooling and heating treatment of a continuously cast strand in which the temperature difference between the surface and central liquid of the castings is kept from becoming excessively great.
Japanese Unexamined Patent Publication No. 54-4224 discloses a method for enhancing the toughness of steels by cooling, during the casting of steels.
In this method, the cooling at a rate of 80.degree. C./min or more at a temperature range directly before the initiation of .delta..fwdarw..gamma. transformation down to the single .gamma.-phase temperature is allegedly effective for enhancing the toughness. It is, however, difficult to lessen the central segregation by such rapid cooling. The patent publication discloses that, in the conventional castings of a continuous casting, the cooling rate is from 15.degree. to 40.degree. C./minute in a temperature range of from 1450.degree. C. to 1200.degree. C. The part of the casting to which the cooling rate corresponds is not specifically shown in the patent publication but is construed, from general knowledge, to be that at the 1/4 thickness of the casting. In addition, the above cooling rate is construed to be an estimated value based on a calculation of the heat transfer.
The cooling rate at the 1/2 thickness of a casting, where the segregation is serious, is greater than that at the 1/4 thickness, at a time directly after the completion of solidification. For example, as shown in the Iron and Steel Handbook, 3rd Edition, Vol. 2, p 621, FIG. 12, 21 (edited by The Iron and Steel Institute of Japan), the cooling rate at the 1/2 thickness of a casting is 60.degree. C./min in a temperature range from 1480.degree. to 1400.degree. C. This cooling rate at the 1/2 thickness of blooms and billets with a small cross-section is considerably greater than the 15 to 40.degree. C./min mentioned above.
It is impossible in such rapid cooling to mitigate central segregation by utilizing solid-state diffusion after solidification and .delta..fwdarw..gamma. transformation.